This invention relates to a process for preparing peroxyborate agglomerates having an active oxygen content of at least 14% by weight, in which amorphous primary particles composed of peroxyborate prepared by a spray drying process are subjected to a compacting pressing-agglomeration; to the peroxyborate agglomerates prepared by this process, and to granular material which can be obtained therefrom.
As used herein, the term "peroxyborates" refers to boron oxygen compounds essentially free from water of crystallization and containing peroxy groups, which have a ratio of equivalents of metal cations (which serve to compensate for negative charges in the compound) to equivalents of boron atoms of .ltoreq.1 and a molar ratio of active oxygen atoms (O.sub.2 ; Avox) to metal atoms of .gtoreq.1. The metal cations can be alkali metal cations and/or alkaline earth metal cations. The alkali metal cations, particularly the sodium cation are, however, preferred so that the compounds are then sodium peroxyborates for example. The term sodium peroxyborate includes both conventional sodium perborate monohydrate (Na:B=1; O.sub.2 :Na=1) and superoxidized sodium perborates having a high active oxygen content (O.sub.2 :Na .gtoreq.1) and perborates with a molar ratio of Na:B (.ltoreq.1, which differs from the conventional Na:B ratio (=1:1), for example perborax in which Na:B=0.5.
Sodium peroxyborates are obtained as solids from aqueous reaction solutions/suspensions by reacting boron-oxygen compounds, after adjusting to the Na:B molar ratio desired in the product, with hydrogen peroxide according to various processes, e.g. by crystallization and optional subsequent drying in order to remove water of crystallization or by direct drying of the reaction solutions/suspensions by vacuum drying, roller drying or spray drying processes. They are used either directly or after further treatment, e.g. agglomerative granulation, for bleaching, disinfecting and as washing agents.
A number of processes for preparing sodium peroxyborates are already known in the art. U.S. Pat. No. 3,914,380 and British Patent No. GB 1,520,127 describe the preparation of attrition-resistant sodium perborate monohydrate (PBS-1) from sodium perborate tetrahydrate (PBS4) obtained by crystallization in a fluid bed dryer, while a relative humidity of 40 to 80%, preferably 50-70%, is maintained in the air surrounding the crystal granules, and while the temperature of the exhaust air is adjusted to at least 60.degree. C. such that partial melting of the perborate is achieved.
German Published Application No. DE 38 04 509 describes a two-stage preparation of sodium perborates, preferably sodium perborate monohydrate. In a first process stage, a very fine perborate monohydrate is initially obtained by spray drying aqueous sodium-, boron- and active oxygen-containing reaction mixtures of the kind also used for preparing sodium perborate tetrahydrate by crystallization. This monohydrate, however, still has unsatisfactory storage stability. This disadvantage is overcome in the above-mentioned patent application by subjecting the very fine primary particles obtained by spray drying to agglomerative granulation in the presence of water in a second process stage immediately after they leave the spray drying zone. The amorphous primary particles are converted to granules of perborate with an at least partially crystalline structure. Spray dryers with an integrated fluidized bed (fluid bed spray dryer FSD) or spray dryers in combination with cylindrical intensive mixers with a fluidized bed device are proposed for carrying out the process.
A fluid bed spray granulation process for preparing attrition-resistant sodium perborate monohydrate granules is proposed in U.S. Pat. No. 4,115,519. To this end, an aqueous solution containing 5 to 75% by weight hydrogen peroxide and an aqueous solution containing 5 to 40% by weight sodium metaborate are sprayed in a fluid bed dryer at a temperature ranging from ambient temperature to 100.degree. C. onto nuclei with smaller dimensions than those of the granular particles to be prepared, and the water present in the aqueous solutions is evaporated by the fluidizing gas introduced into the fluid bed dryer.
The preparation of solid, superoxidized sodium perborate containing more than 16% by weight active oxygen and less than 1.4 atoms of hydrogen per active oxygen atom is described in U.S. Pat. No. 4,185,960. To this end, an at least 30% by weight hydrogen peroxide solution and an aqueous sodium metaborate solution are sprayed simultaneously into a fluid bed dryer. The fluid bed dryer contains nuclei whose dimensions are smaller than those of the perborate particles to be prepared, and the hydrogen peroxide and sodium metaborate are introduced into the fluid bed in a molar ratio which is greater than 1.12 (fluid bed spray granulation process FSG).
In a process according to U.S. Pat. No. 4,681,748, superoxidized sodium perborates are prepared by drying a solution or suspension of sodium metaborate and hydrogen peroxide with a borate concentration (calculated as B.sub.2 O.sub.3) of 90 to 130 g/1 and a molar ratio of H.sub.2 O.sub.2 to B.sub.2 O.sub.3 of 2.0:1 to 4.2:1 in a spray dryer at an exhaust air temperature of 55.degree. to 120.degree. C. (preferably 60.degree. to 95.degree. C.) and recovering the resulting spray-dried material.
U.S. Pat. No. 2,491,769 describes the preparation of sodium peroxyborates with Na:B molar ratios of 0.53 to 0.97, preferably 0.8 to 0.95. To this end, aqueous mixtures containing boron-oxygen and sodium compounds are reacted with hydrogen peroxide while maintaining the given Na:B ratios, and the solid sodium peroxyborates are subsequently recovered by rapid drying of the reaction mixture. Roller dryers are used for drying, but the possibility of vacuum drying or spray drying is also mentioned.
German Patent No. DE 901,287 also describes sodium perborates with Na:B molar ratios of less than 1, particularly less than 0.7. The products are obtained by adding boric acid and hydrogen peroxide to a sodium borate in ratios such that the end product contains active oxygen according to a molar ratio of H.sub.2 O.sub.2 :Na of 0.5:1 to 4.1:1 and such that the molar ratio of Na:B in the end product is less than 0.5:1. The resulting slurry or solution is dried by evaporation. Drying is preferably carried out under reduced pressure and at a somewhat elevated temperature. The purpose of the described process is to prepare sodium peroxyborate products that can be mixed in the dry state with boric acid to yield dry powders so that products are obtained which have an acid or essentially neutral reaction and, in addition, have a buffering effect.
German Published Application No. DE 1 112 502 describes a process for preparing perborax (Na:B=0.5) by spray drying a reaction mixture containing borax and hydrogen peroxide. Products with active oxygen contents of 15 to 18% by weight can be obtained, but a perborax with an active oxygen content of 9 to 12% by weight is preferentially prepared. In order to increase the rate of dissolution of the perborax which is obtained, it is moreover desirable to incorporate an even fine distribution of 5 to 15% by weight salts which solidify in crystalline form in the reaction mixture before spray drying.
The processes of the prior art have a number of disadvantages. They include, for example, a relatively large number of separate process stages. For example, during the preparation of PBS-1 from PBS-4, the perborate tetrahydrate must initially be obtained by crystallization and subsequently passed to a drying stage in order to remove the water of crystallization. The perborate products obtained by spray drying are usually very fine particles and therefore cannot be used immediately. Although these very fine particles can be converted to larger particles according to the state of the art by agglomerative granulation in the presence of water, active oxygen losses can occur during this process. Such an after-treatment requires the presence of granulating aids, e.g. binders and solvents, such as water, as a result of which an additional subsequent drying stage is required. Moreover, in all the sodium peroxyborates of the prior art, the solubility, the rate of dissolution, and the resistance of the particles to caking are unsatisfactory or require further improvement. There remained, therefore, a need for a simple, direct process for preparing peroxyborate agglomerates, particularly sodium peroxyborate granules, having a high active oxygen content, having particle sizes and bulk densities convenient for use, and having good solubility and a high rate of dissolution.